Field of the Invention
The present invention relates to communications equipment and, more specifically but not exclusively, to antenna arrays for base stations in cellular communications networks.
Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
Many modern base station antennas (BSAs) need to be multi-carrier and multi-operator and need to support different communication standards and different frequency ranges. One BSA can cover two or more relatively wide frequency bands (e.g., 698 MHz to 960 MHz plus 1710 MHz to 2690 MHz). To work with different standards and/or with different operators, a wideband BSA can be integrated with distributed filters.
Antenna arrays with distributed filters are known in the art. See, e.g., U.S. Pat. No. 6,208,299 B1 (the '299 patent). The advantage of the '299 patent is the possibility to obtain the same beamwidth for different frequency sub-bands. But the '299 patent does not allow independent beam tilt (or beam scanning) for its different sub-bands.
Antennas with distributed filters and independent (phased-array) beam tilt for each sub-band are known in the market. See, e.g., the Type No. 80010668 BSA with Adjustable Electronic Downtilt unit from Kathrein of Rosenheim, Germany.
FIG. 1 shows a simplified block diagram of a prior-art four-element antenna system 100 for supporting communications in two different sub-bands f1 and f2 with independent beam tilt. Independent beam tilt is provided by two phase shifter networks PS1, PS2, combining phase shifters and power dividers. Note that each sub-band may have different frequency ranges within the sub-band for uplink and downlink transmissions. Each antenna element 102 has its own nearby, dedicated cavity diplexer CD that combines downlink (i.e., outgoing) signals for the two different sub-bands for transmission from the corresponding antenna element 102 and separates uplink (i.e., incoming) sub-band signals received at the corresponding antenna element 102 for application to respective phase shifter networks PS1 and PS2, which provide independent beam tilt for the two sub-bands f1 and f2. The multi-cavity diplexers, connected to antenna elements 102 and phase shifter networks PS1, PS2, are used to get desirable inter-band (e.g., inter-system) isolation greater than about 30 dB between port1 and port2. Unfortunately, multi-cavity diplexers are expensive and can represent about 80% of the total antenna system cost.
Another disadvantage of prior-art antenna system 100 of FIG. 1 is that different sub-bands (having different frequencies) will have different beamwidths, because antenna beamwidth is in inverse proportion to frequency. In some cases, the beamwidths for higher-frequency wireless bands (for example, for 2.6 GHz or 3.5 GHz) become too narrow and cannot illuminate the required geographic zone. As a result, there can be a limit on the minimal-elevation beamwidth (e.g., the half-power beamwidth cannot be less than 4.5 degrees). On another hand, if the beamwidth of an antenna is too wide, then there can be too much interference with other cells. The unwanted signals reduce the signal-to-noise ratio, forcing the use of fewer efficient transmission modulations (from 64 QAM to 8 QAM or even worse QSPK) in areas where the signal-to-noise ratio is not big enough.